Gynot,
I have sandy soil too.
I would plant with lots of composted cow manure, ALOT of it, and mulch it real good. Not up to the roots, but mulch it at least 3 inches deep.
Water it every day for a wekk after you plant it, then at least twice a week in the ehat of the summer, mabey more often than that.
Hibiscus loves water and sun, and since water runs right through sandy soil, the composted manure will build the soil up to hold some water for you.
Personally, I dug a giant hole, I bought top soil from the nbursery, mixed in lots of manure, and planted my plant in that. Then I mulched.
That is how sandy my soil is.